This adds the following:
- A detect-secrets dependency and a related script
(`script/detect_secrets`) to find and alert developers to secrets
added to the code. By default, the script will search staged and new,
unstaged files. It can optionally search only staged files.
- A whitelist, `.secrets.baseline`, that tracks instances of secrets or
false positives already in the repo.
- Modifies `script/test` to detect secrets as part of the test suite.
- Updates to the README regarding the use of detect-secrets.
